This invention relates to a brake booster for boosting brake operational force by utilizing gaseous pressure, and more particularly to an improvement of a brake booster of stroke-enlarging type.
A stroke-enlarging type brake booster herewith referred to is a brake booster in which a power piston disposed in a booster casing in such a state as to divide the interior thereof into two chambers for being actuated by the pressure difference in those two chambers is made relatively movable to a control piston having a control valve or a valve mechanism for controlling the above-mentioned pressure difference due to operation of an input member and a transmission mechanism for transmitting the operational force applied on the input member toward an output member, whereby increasing of a stroke at the output member is attempted by the relative movement of the power piston to the control piston. A booster of this type is characterized in producing a larger output stroke than the input stroke applied, while the output stroke is the same as the input stroke or less in ordinary boosters. A brake booster of stroke-enlarging type has been conventionally somewhat defective in rapidly increasing only the pedal stroke, an input stroke, without increasing the braking force at all at a critical point, which corresponds to an upper limit of an assisting force applied on the power piston by the pressure difference on either side of the power piston.
The applicants of this invention developed some time ago a brake booster, for eliminating the above-mentioned disadvantage, having a highly reliable anti-backdown mechanism capable of effectively preventing the retracting of the power piston at the critical point, while being simple in structure and sure in keeping away delay in action. This brake booster having eliminated the troublesome idle stroke and the resultant brake feeling deterioration was filed as a patent application in the United States with the application No. 223,565, now U.S. Pat. No. 4,387,625. This anti-backdown mechanism includes a plurality of balls rotatably held by a ball retainer and disposed between the power piston and the control piston. A ball accommodating recess is formed on the external surface of the control piston. The bottom surface of the recess is made so slant as to gradually approach the axis of the control piston along the forward direction. A ball driving surface is formed on the internal surface of the power piston. The ball driving surface is made so slant as to depart from the axis of the power piston along the forward direction for being able to advance the balls forwardly. A part of the ball driving surface where the same is contacted with the balls when the power piston is farthest advanced relative to the control piston is made more gently slope than the rest part thereof. The forward end of the ball retainer abuts on a first input portion of a reaction lever, whereby the output force of the power piston is transmitted via the balls and the ball retainer to the reaction lever and the reactionary force transmitted from the output member via the reaction lever to the ball retainer is, when the power piston is at the farthest position, is transmitted to both the control piston and the power piston through an engagement between the balls and the bottom surface of the ball accommodating recess and the gentle slope portion of the ball driving surface.
This type of the brake booster is, owing to its structure, effective in its output stroke enlarging function until the power piston is locked on the control piston upon having reached its farthest position. Due to lack of uniformity of clearance in the brake which is connected to the output member, possible premature extinction of the brake clearance which may happen prior to reaching of the power piston to the farthest position will cause a strong brake operation exceeding the expectation or wish of the driver because of the continued stroke enlarging effect. An attempted design of a booster making the power piston reach the farthest position when the smallest expected brake clearance has disappeared turned out unsuccessful, because the brake clearance remained unvanished in ordinary cases even after termination of the output stroke enlarging action. It hindered satisfactory enjoyment of the output stroke enlarging effect, contrary to the expectation.